[Background Sync] Remove extra object from unittest.

Remove an extra BackgroundSyncManager object from BackgroundSyncManager
unittests.

Bug: 933849
Change-Id: I4f3e5d598c1778d20ae33f8921b0fd85da535a6b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1675551
Commit-Queue: Mugdha Lakhani <nator@chromium.org>
Reviewed-by: Rayan Kanso <rayankans@chromium.org>
Reviewed-by: Peter Beverloo <peter@chromium.org>
Cr-Commit-Position: refs/heads/master@{#672144}
2 files changed